The migration of Germanic tribes into western Europe led to the decline of the Roman Empire. The spread of Christianity meant that people's loyalty was first to the Church, before the Empire.
The rapid spread of Christianity coincided with the decline of the Roman Empire, and some historians believe that Christianity contributed to that decline.
Nero's roman empire
Marrocos infantry and chialvary captured Sangais Empire
The emperor tried to divide regions of territory out to his sons. These empires did not follow cultural, ethical ties, and did not have local nobility support. This caused the decline of the Frankish empire.
The decline of the Roman Empire did not favour the spread of Christianity, it was three dynasties of emperors which were Christians and favoured Christanity. One of these emperors also persecuted paganism.
Historians generally agree that the Roman Empire was already in decline before Christianity became dominant during the fourth century CE. Some say that Christianity was the dominant reason for its subsequent rapid decline, at least in the west, while others say that it was only a contributing factor, although an important one. Christianity did not actually strengthen the empire.